Valve actuation in an internal combustion engine is required in order for the engine to produce positive power. During positive power operation, one or more intake valves may be opened to admit fuel and/or air into a cylinder for combustion. One or more exhaust valves may be opened to allow combustion gas to escape from the cylinder. Intake, exhaust, and/or auxiliary valves also may be opened during positive power at various times to recirculate gases for improved emissions.
Valve actuation may also be required to produce auxiliary engine valve actuations, such as engine braking, for example. During compression-release type engine braking, the exhaust valves may be selectively opened to convert, at least temporarily, a power producing internal combustion engine into a power absorbing air compressor. As a piston travels upward during its compression stroke, the gases that are trapped in the cylinder may be compressed. The compressed gases may oppose the upward motion of the piston. As the piston approaches the top dead center (TDC) position, at least one exhaust valve may be opened to release the compressed gases in the cylinder to the exhaust manifold, preventing the energy stored in the compressed gases from being returned to the engine on the subsequent expansion down-stroke. In doing so, the engine may develop retarding power to help slow the vehicle down. An example of a prior art compression release engine brake is provided by the disclosure of the Cummins, U.S. Pat. No. 3,220,392 (November 1965), which is hereby incorporated by reference.
During bleeder type engine braking, in addition to, and/or in place of, the main exhaust valve event, which occurs during the exhaust stroke of the piston, the exhaust valve(s) may be held slightly open during the remaining three engine cycles (full-cycle bleeder brake) or during a portion of the remaining three engine cycles (partial-cycle bleeder brake). The bleeding of cylinder gases in and out of the cylinder may act to retard the engine. Usually, the initial opening of the braking valve(s) in a bleeder braking operation is in advance of the compression TDC (i.e., early valve actuation) and then lift is held constant for a period of time. As such, a bleeder type engine brake may require lower force to actuate the valve(s) due to early valve actuation, and generate less noise due to continuous bleeding instead of the rapid blow-down of a compression-release type brake.
Another auxiliary engine valve actuation is exhaust gas recirculation (EGR), during which a portion of the exhaust gases may flow back into the engine cylinder during positive power operation. EGR may be used to reduce the amount of NOx created by the engine during positive power operations. An EGR system can also be used to control the pressure and temperature in the exhaust manifold and engine cylinder during engine braking cycles. Generally, there are two types of EGR systems, internal and external. External EGR systems recirculate exhaust gases back into the engine cylinder by direct passage from the exhaust manifold to the intake manifold and then back into the cylinder through an intake valve(s). Internal EGR systems recirculate exhaust gases back into the engine cylinder from the exhaust manifold through an exhaust valve(s) and potentially back to the intake manifold from the engine cylinder through an intake valve(s). Embodiments of the present invention primarily concern internal EGR systems.
Still another auxiliary engine valve actuation is brake gas recirculation (BGR), during which a portion of the exhaust gases may flow back into the engine cylinder during engine braking operation. Recirculation of exhaust gases back into the engine cylinder during the intake stroke, for example, may increase the mass of gases in the cylinder that are available for compression-release braking. As a result, BGR may increase the braking effect realized from the braking event.
In many internal combustion engines, the intake and exhaust valves may be opened and closed by fixed profile cams, and more specifically by one or more fixed lobes that are an integral part of each of the cams. Benefits such as increased performance, improved fuel economy, lower emissions, and/or better vehicle driveability and braking may be obtained if the intake and exhaust valve timing and/or lift can be varied. The use of fixed profile cams, however, can make it difficult to adjust the timings and/or amounts of engine valve lift in order to optimize them for various engine operating conditions, such as different engine speeds.
One method of adjusting valve timing and lift, given a fixed cam profile, has been to provide a “lost motion” device in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, or other linkage assembly in the valve train. In a lost motion system, a cam lobe may provide the “maximum” lift motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage, intermediate of the valve to be opened and the cam providing the maximum motion, to selectively extend the duration of the maximum lift past the duration provided by the cam and/or subtract or lose part or all of the lift provided by the cam.
This variable length system (or lost motion system) may, when expanded fully, transmit all of the cam motion to the valve and even extend the duration of the valve event beyond that normally provided by the cam, and when contracted fully, transmit none or a minimum amount of the cam motion to the valve. Examples of lost motion systems and methods are provided in Hu, U.S. Pat. Nos. 5,537,976 and 5,680,841, which are assigned to the same assignee as the present application and which are incorporated herein by reference.
A second example of a lost motion valve actuation system is disclosed in published U.S. patent application Ser. No. 11/123,063 (“the '063 application”), filed May 6, 2005, and published on Jan. 12, 2006 as publication number US 2006/0005796, which is incorporated herein by reference. The '063 application discloses a valve actuation system that utilizes a primary rocker arm and an auxiliary rocker disposed adjacent to each other on a rocker arm shaft. The primary rocker arm may actuate an engine valve for primary valve actuation motions, such as main exhaust events, in response to an input from a first valve train element, such as a cam lobe. The auxiliary rocker arm may receive one or more auxiliary valve actuation motions, such as for engine braking, exhaust gas recirculation, and/or brake gas recirculation events, from a second valve train element, such as a second cam lobe. An adjustable hydraulic actuator piston may be disposed between the auxiliary rocker arm and the primary rocker arm. The actuator piston may be selectively locked into an extended position between the primary and auxiliary rocker arms so as to selectively transfer one or more auxiliary valve actuation motions from the auxiliary rocker arm to the primary rocker arm and thereafter to the engine valve. The hydraulic actuator piston may be preferably provided in either the primary or the auxiliary rocker arm.
While various embodiments of the present invention may be used particularly in connection with a primary rocker arm and auxiliary rocker arm system such as that disclosed in the '063 application, no embodiments should be limited to use with only such systems. Thus, the hydraulic fluid systems, and methods of operation thereof, which are disclosed in the present application may provide improved valve actuation for compression-release engine braking, bleeder type engine braking, exhaust gas recirculation, brake gas recirculation, and/or any other auxiliary valve events carried out by a system such as that disclosed in the '063 application. More specifically, various embodiments of the present invention may reduce or eliminate transient loads experienced by valve train elements, such as engine valves, rocker arms, rocker shafts, push tubes and/or cams, when the auxiliary rocker arm first engages and disengages the primary rocker arm for auxiliary engine valve events, such as engine braking, etc.
Additional advantages of the invention are set forth, in part, in the description that follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.